1. Field of the Invention
This invention relates to an ultrasonic diagnostic apparatus and an image filtering method of the same. More particularly, the present invention relates to an image filtering method of an ultrasonic diagnostic apparatus adapted to forming an image according to an ultrasonic Doppler signal obtained from an object of examination.
2. Description of the Related Art
Among various filter processes for processing raster images, the process using a median filter (a median extracting filter) is known to be effective for removing noises, reducing missing pixels and smoothing, while maintaining the resolution and the boundaries of objects in the image. The color Doppler method currently being used for ultrasonic diagnostic processes represents a typical application of median filters.
The color Doppler method is a technique of scanning a living body along a predetermined cross section by means of an ultrasonic pulse, capturing the Doppler frequency deviation that arises as a function of the speed of a moving reflecting object (the speed of flowing blood) when an ultrasonic is irradiated onto the moving reflecting object such as blood (blood cells) and visualizing it as an image. While an excellent gauging accuracy (particularly a low running speed sensing ability), a remarkable temporal resolution and also a good spatial resolution are required to improve the diagnostic accuracy of the color Doppler method, it is difficult to highly satisfy the requirements at the same time. For this reason, a technique of smoothing the boundary lines of blood vessels and blood flow distributions, removing noises and reducing image skips (so called a black failure pattern) that arise due to the phenomenon of interference of ultrasonic by means of a two-dimensional median filter (median extracting filter) is proposed to cope with the problem of degradation of spatial resolution (see, for example, Jpn. Pat. Appln. Publication No. 2000-262520).
However, a median filtering process is intrinsically an operation of sorting (rearranging) data and involves a large number of computations for comparisons and replacements. Thus, various techniques have been proposed to reduce the number of computations of a sorting operation in a median filtering process.
For example, a technique of firstly sorting each of the columns of the data of three rows and three columns, then sorting each of the rows and finally sorting three data on the diagonals to acquire the median value thereof (see Jpn. Pat. Appln. Publication No. 11-149554). “Fast Algorithms for Median Filtering”, Intel Corporation, 2001 shows a method of sorting data up to five rows and five columns at maximum in stead of three rows and three columns.
The algorithms described in Jpn. Pat. Appln. Publication No. 11-149554 and “Fast Algorithms for Median Filtering”, Intel Corporation, 2001 cited above are effective when computing a plurality of data simultaneously by means of a single instruction multiple data (SIMD) processor. However, they still require a large number of computations for a median filtering process.